[clang] [lld] [llvm] [LLVM][WebAssembly] Implement branch hinting proposal (PR #146230)

[Lukas Döllerer via llvm-commits]

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@@ -2608,6 +2612,21 @@ void WebAssemblyCFGStackify::rewriteDepthImmediates(MachineFunction &MF) {
         Stack.push_back(std::make_pair(&MBB, &MI));
         break;
 
+      case WebAssembly::BR_IF: {
+        // this is the last place where we can easily calculate the branch
+        // probabilities. we do not emit scf-ifs, therefore, only br_ifs have
+        // to be annotated with branch probabilities.
+        if (MF.getSubtarget<WebAssemblySubtarget>().hasBranchHinting() &&
+            MI.getParent()->hasSuccessorProbabilities()) {
+          const auto Prob = MBPI->getEdgeProbability(
+              MI.getParent(), MI.operands().begin()->getMBB());
+          WebAssemblyFunctionInfo *MFI = MF.getInfo<WebAssemblyFunctionInfo>();
+          assert(!MFI->BranchProbabilities.contains(&MI));
+          MFI->BranchProbabilities[&MI] = Prob;
+        }
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Lukasdoe wrote:

Update: The changes in the `CFGGraphStackify` pass are now completely reverted. Branch hints are only generated based on `br_if` instructions encountered during ASM printing, mostly based on @kripken 's earlier contribution. This change significantly reduces the memory overhead (and probably runtime overhead), since we do not store branch hints for branches that are removed by passes after `CFGStackify`.

https://github.com/llvm/llvm-project/pull/146230